The present invention generally relates to an optimal write conductor layout in a magnetic random access memory (MRAM). More particularly, the present invention relates to a write conductor layout wherein the write conductors are in generally parallel alignment as they cross the MRAM memory cell to provide improved half-select margins and to reduce write current requirements.
An MRAM device includes an array of memory cells. The typical magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization is fixed or xe2x80x9cpinnedxe2x80x9d in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer and the magnetic film which is pinned may be referred to as a reference layer.
Conductive traces (commonly referred to as word lines and bit lines) are routed across the array of memory cells. Word lines extend along rows of the memory cells, and bit lines extend along columns of the memory cells. Located at each intersection of a word line and a bit line, each memory cell stores the bit of information as an orientation of a magnetization. Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as its easy axis. External magnetic fields are applied to flip the orientation of magnetization in the data storage layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer, depending on the desired logic state.
The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent logical values of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. The orientation of magnetization of a selected memory cell may be changed by supplying current to a word line and a bit line crossing the selected memory cell. The currents create magnetic fields that, when combined, can switch the orientation of magnetization of the selected memory cell from parallel to anti-parallel or vice versa.
A selected magnetic memory cell is usually written by applying electrical currents to the particular word and bit lines that intersect at the selected magnetic memory cell. Typically, an electrical current applied to the particular bit line generates a magnetic field substantially aligned along the easy axis of the selected magnetic memory cell. The magnetic field aligned to the easy axis may be referred to as a longitudinal write field. An electrical current applied to the particular word line usually generates a magnetic field substantially perpendicular to the easy axis of the selected magnetic memory cell.
Preferably, only the selected magnetic memory cell receives both the longitudinal and the perpendicular write fields. Other magnetic memory cells coupled to the particular word line usually receive only the perpendicular write field. Other magnetic memory cells coupled to the particular bit line usually receive only the longitudinal write field.
The magnitudes of the longitudinal and the perpendicular write fields are usually chosen to be high enough so that the selected magnetic memory cell switches its logic state when subjected to both longitudinal and perpendicular fields, but low enough so that the other magnetic memory cells which are subject only to either the longitudinal or the perpendicular write field do not switch. An undesirable switching of a magnetic memory cell that receives only the longitudinal or the perpendicular write field is commonly referred to as half-select switching.
Manufacturing variation among the magnetic memory cells often increases the likelihood of half-select switching. For example, manufacturing variation in the longitudinal or perpendicular dimensions or shapes of the magnetic memory cells may increase the likelihood of half-select switching. In addition, variation in the thicknesses or the crystalline anisotropy of data storage layers may increase the likelihood of half-select switching. Unfortunately, such manufacturing variation decreases the yield in manufacturing processes for magnetic memories and reduces the reliability of prior magnetic memories.
Because the word lines and the bit lines operate in combination to switch the orientation of magnetization of the selected memory cell (i.e., to write the memory cell), the word lines and bit lines can be collectively referred to as write lines. Additionally, the write lines can also be used to read the logic values stored in the memory cell.
FIG. 1 illustrates a top plan view of a simplified prior art MRAM array 100. The array 100 includes memory cells 120, word lines 130, and bit lines 132. The memory cells 120 are positioned at each intersection of a word line 130 with a bit line 132. Most commonly, the word lines 130 and bit lines 132 are arranged in orthogonal relation to one another and the memory cells 120 are positioned in between the write lines (130,132), is illustrated in FIG. 1b. For example, the bit lines 132 can be positioned above the memory cells 120 and the word lines 130 can be positioned below.
FIGS. 2a through 2c illustrate the storage of a bit of data in a single memory cell 120. In FIG. 2a, the memory cell 120 includes an active magnetic data film 122 and a pinned magnetic film 124 which are separated by a dielectric region 126. The orientation of magnetization in the active magnetic data film 122 is not fixed and can assume two stable orientations is shown by arrow M1. On the other hand, the pinned magnetic film 124 has a fixed orientation of magnetization shown by arrow M2. The active magnetic data film 122 rotates its orientation of magnetization in response to electrical currents applied to the write lines (130,132, not shown) during a write operation to the memory cell 120. The first logic state of the data bit stored in as memory cell 120 is indicated when M1 and M2 are parallel to each other as illustrated in FIG. 2b. For instance, when M1 and M2 are parallel a logic xe2x80x9c1xe2x80x9d state is stored in the memory cell 120. Conversely, a second logic state is indicated when M1 and M2 are anti-parallel to each other as illustrated in FIG. 2c. Similarly, when M1 and M2 are antiparallel a logic xe2x80x9c0xe2x80x9d state is stored in the memory cell 120. In FIGS. 2b and 2c the dialectic region 126 has been omitted. Although FIGS. 2a through 2c illustrate the active magnetic data film 122 positioned above the pinned magnetic film 124, the pinned magnetic film 124 can be positioned above the active magnetic data film 122.
The resistance of the memory cell 120 differs according to the orientations of M1 and M2. When M1 and M2 are anti-parallel, i.e., the logic xe2x80x9c0xe2x80x9d state, the resistance of the memory cell 120 is at its highest. On the other hand, the resistance of the memory cell 120 is at its lowest when the orientations of M1 and M2 are parallel, i.e., the logic xe2x80x9c1xe2x80x9d state. As a consequence, the logic state of the data bit stored in the memory cell 120 can be determined by measuring its resistance. The resistance of the memory cell 120 is reflected by a magnitude of a sense current 123 (referring to FIG. 2a) that flows in response to read voltages applied to the write lines (130,132).
In FIG. 3, the memory cell 120 is positioned between the write lines (130,132). The active and pinned magnetic films (122,124) are not shown in FIG. 3. The orientation of magnetization of the active magnetic data film 122 is rotated in response to a current Ix that generates a magnetic field Hy and a current Iy that generates a magnetic field Hx. The magnetic fields Hx and Hy act in combination to rotate the orientation of magnetization of the memory cell 120.
As can be seen, there is a need for an NRAM array which provides improved half-select margins. Additionally, there is a need to reduce power consumption in the MRAM memory cell by reducing the magnitude of the write currents necessary to rotate the orientation of magnetization of the data storage layer during write operations to the memory cell. For example, reduced power can result in a reduction in the waste heat generated by an electronic device incorporating the MRAM memory. Moreover, for portable devices, it is desirable to reduce power consumption to extend battery life.
The present invention is a magnetic random access memory array and a unique write conductor layout structure for a magnetic memory cell. The magnetic memory cell includes a data storage layer having an easy axis. A first conductor is positioned on a first side of the data storage layer, wherein a portion of the first conductor has an angle of orientation that is perpendicular to the easy axis. A second conductor is positioned on a second side of the data storage layer, wherein a portion of the second conductor has an angle of orientation that is perpendicular to the easy axis. The MRAM array provides improved half-select margins and reduces power consumption in the MRAM memory cell by reducing the magnitude of the write currents necessary to rotate the orientation of magnetization of the data storage layer during write operations to the memory cell.